The invention relates to fastening of a bearing bolt in a cylindrical housing of a roller tappet actuated in the direction of a longitudinal axis, especially for a tappet push-rod valve drive of an internal combustion engine. The bearing bolt is supported in bore holes of the housing, which extend in a transverse plane to the roller tappet, perpendicular to the longitudinal axis and connected to the housing with a positive and/or non-positive fit through material deformation as a result of end-side swaging of the bearing bolt in the direction of the housing.
The fastening of bearing bolts in housings of roller tappets by means of end-side swaging or stamping of the bearing bolt with the housing has been known to someone skilled in the art for a long time as a time-saving and cost-effective measure to connect the bearing bolt to the housing of the roller tappet in a functionally reliable and long-term manner. For example, U.S. Pat. No. 6,196,175 B1 shows a roller tappet valve drive with a roller tappet that is here embodied as a switchable roller tappet for deactivating the gas-exchange valve. A good view of a typical swaging pattern can be seen at the end of the bearing bolt. This pattern results from the material-deforming effect of a stamping tool in the region of the radial end of the bearing bolt. Such a swaging pattern often has the shape of a continuous circular groove, which is formed in the end side of the bearing bolt by the stamping tool, which wobbles about the axis of the bearing bolt, for example, under the application of force. As described in the cited document, this material deformation can also be composed of numerous circular arc-shaped segments, which alternate with non-deformed, but relatively short sections.
Fixation of the bearing bolt embodied in this way can be disadvantageous for several reasons. First, the bearing bolt is frequently used as a support for a highly stressed cam roller of a roller tappet in internal combustion engines with a roller tappet valve drive and underlying camshaft. The diameter of such a roller tappet is typically based directly on the diameter and width of the cam roller and is kept as small as possible for reasons of the moving valve drive mass. This has the result that the cam roller is held either in a roller pocket of the roller tappet, wherein the roller pocket is closed on the periphery but locally has very thin walls, or is arranged merely between two axial connecting pieces of the housing for supporting the bearing bolt.
However, in both cases the radial inherent stability of the housing of the roller tappet is considerably limited in the region of the cam roller. In this respect, material deformation, which extends continuously past the end periphery of the bearing bolt or which is distributed uniformly, with a high percentage of deformed segments has the result that the originally cylindrical housing in the region of the cam roller is deformed to be unacceptably high and typically oval after the swaging process due to the material deformation extending in the radial direction of the housing.
Such shape deformation can be problematic, especially in the roller tappet valve drives of the type noted above, whose roller tappets are typically manufactured from steel and supported in a guide of the internal combustion engine composed from gray iron. This is due to the very similar thermal expansion coefficients of steel and gray iron, so that advantageously a largely temperature-independent and thus extremely small guidance play of the roller tappet in its guide can be realized. However, shape deformation of the housing with already very small deviations from the cylindrical form can simultaneously have the result that the roller tappet can be installed during the assembly process either not at all or only under tamping into the guide, or that the roller tappet jams in the guide when the engine is running. Possible consequences of the latter case include, in the best case, a gas-exchange valve that no longer closes completely and, in the worst case, engine damage due to mechanical valve-drive stress or due to a piston colliding with an open gas-exchange valve.
Another disadvantage of the known connection of the bearing bolt by means of swaging is that continuous or uniformly distributed material deformation with a high percentage of deformed segments leads to minimal local material deformation of the bearing bolt for constant stamping forces, whereby the security against detachment of the connection of the bearing bolt to the housing of the roller tappet is reduced. In addition, material deformation, as embodied in the cited document as a plurality of segments alternating with non-deformed sections, is rated as unfavorable with reference to the press fit between the end section of the bearing bolt and the associate bore hole of the housing. The cause of this is a non-uniform force distribution in the force fit. The non-uniform force distribution can lead to excessive material stresses in the region of the load zone formed on the bearing bolt due to the introduction of forces via the cam roller and consequently to a flow of bearing bolt material in the force fit in the region of the load zone, because the bearing bolt at the end sections exhibits a relatively low material hardness for the purpose of deformation. In the case of such material flow, the security against detachment of the fixation of the bearing bolt is also reduced. A connection that is no longer effective typically leads to friction-generating contact of the bearing bolt with the guide due to the bearing bolt coming out of the housing at the sides. This can lead to destruction of the guide sleeve and subsequent jamming of the roller tappet in the guide in a short time with the consequences and damages explained above.